Polyolefins prepared by catalytic polymerisation or copolymerisation of olefins, for example, ethylene, propylene or higher olefins such as C4 to C12 alpha-olefins, are generally subjected to a process for removal of the bulk of the unreacted monomer before being processed into useful articles. Such processes for removal of unreacted monomer generally involve a monomer separation and recovery procedure wherein the bulk of the unreacted monomer associated with the polyolefin product is separated therefrom when the polyolefin is first removed from the polymerisation reactor. Processes for this initial monomer separation and recovery depend on the particular technology being employed for the polymerisation reaction. For example, in the gas phase (co)polymerisation of olefins, the polyolefin product is normally a fine powder fluidised by, or stirred in, an atmosphere comprising the gaseous monomer(s). Monomer may be separated and recovered from the gas phase process, for example, by isolating a continuous stream of particulate polymer product associated with at least some gas, and optionally some liquid, comprising unreacted monomer; reducing the pressure and recycling the volatile components to the reactor; and purging the polymer component with inert gas, for example, nitrogen or carbon dioxide.
Thus the volatile materials referred to throughout this specification can be, for example, the monomer or monomers themselves, oligomers, any solvent or diluent used in the polymerisation, the catalyst materials or products derived therefrom, additives in the polymerisation (e.g. molecular weight regulators), impurities present in any of the materials used in the polymerisation, or materials employed for lubricating moving parts of the reactors. Such volatile substances can also arise from degradation or inter-reaction of the polymerisation components themselves and/or their products. The presence of such volatile substances in the final polymer is generally undesirable and can result, for example, in unwanted odour in articles manufactured therefrom, or can produce taint in foodstuffs packed in containers fabricated from the polymer, or in water from potable water piping systems. The presence of inflammable volatile materials can also present a fire or explosion hazard. Likewise, such volatile materials can have toxic, irritant or other undesirable pharmacological properties which normally render their removal desirable or even essential.
The production of volatile substances in the polymer can also occur during pelletisation of the polymer, for example, by thermal degradation of the polymer itself, or by the degradation of additives employed in the pelletising process.
GB-A-1272778 relates to a process for the removal of volatile constituents from particulate olefin polymers which have been produced by the gas phase polymerisation of the monomers for example of ethylene or propylene, by treating a layer of the polymer, whose particles have a mean diameter of from 10 to 5000 microns, with a stream of inert gas at a temperature of from at least +80° C., to at least 5° C. below the crystallite melting of the polymer in the treatment zone while keeping the layer in vigorous motion.
EP-A-0047077 relates to a process for removing unpolymerised gaseous monomers from solid olefin polymers by conveying the polymer (e.g. in granular form) to a purge vessel, contacting the polymer in the purge vessel with a countercurrent inert gas purge stream to strip away the monomer gases which are evolved from the polymer, and recycling a portion of the resulting inert gas-monomer gas stream to the purge vessel.
The present invention is concerned with an improved method for the removal of traces of volatile material, e.g. unreacted monomer, oligomers or other volatile constituents such as solvent or degradation products, from particulate polymeric materials, especially polymers prepared by the catalytic polymerisation of organic monomers.